Pressure-driven membrane separation processes allow a broad range of neutral and ionic species to be removed from fluids. In order of decreasing pore size, membranes are commonly classified into several categories: microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO). Microfiltration is used for removal of suspended particles having particle sizes greater than 0.1 microns. Ultrafiltration commonly excludes dissolved molecules having molecular weights greater than 5,000 daltons. Nanofiltration membranes pass at least some salts but usually have high retention of organic compounds having molecular weights greater than approximately 200 daltons. Reverse osmosis membranes have high retention of almost all species.
An alternative means of characterizing membranes is by their method of formation. MF and UF membranes can be made by a wide variety of techniques and commercially significant methods include etching, sintering, partial fracture by stretching, and phase-inversion. NF and RO membranes are generally made by either phase inversion or interfacial polymerization. Interfacial polymerization results in a composite structure having a very thin discriminating layer with high selectivity affixed to a porous support and it is by far the dominate procedure for creating NF and RO membranes. Interfacial polymerization can be performed with a wide variety of monomers as is described in U.S. Pat. No. 6,337,018 incorporated herein in its entirety by reference.
NF and RO membranes are most commonly used in applications such as desalination of seawater or brackish water, production of ultrapure water, color removal, waste water treatment, and concentration of liquids for food products. A critical factor in almost all NF and RO applications is that the membrane achieve high rejection of small solute molecules while maintaining high flux.
A spiral wound element is the most common configuration for RO and NF membranes. A classic spiral wound element design is illustrated in FIG. 1. “Feed” liquid flows axially through a feed spacer sheet and exits on the opposite end as “concentrate”. “Permeate” passes under pressure through membrane envelopes and is directed to a permeate collection tube by a permeate carrier sheet. In comparison to alternative configurations (hollow-fiber, plate-and-frame, and tubular modules), spiral wound elements often have a favorable combination of low cost, low polarization, and low pressure drop across the element.
Element performance can be further enhanced with optimal element design. For example, it is possible to simultaneously vary the number of envelopes in an element and their lengths to optimize efficiency. For the same element diameter, increasing the number of envelopes in an element results in less active area since more envelopes means more inactive end regions. However, increasing envelope length (as measured in the direction perpendicular from the axis of collection tube) results in greater pressure drop within the longer permeate carrier sheets and this can also decrease element flow during operation. (An increased pressure drop within the permeate carrier sheets results in less flux—flow per unit of membrane area.) For a given set of conditions, the optimal trade-off between envelope length and the number of membrane envelopes can be made to maximize flow. The same trade-off also impacts solute rejection which is maximized for element designs with a larger number of envelopes and shorter individual envelope lengths. Assuming knowledge of several parameters including expected operating conditions, the required element diameter, the active width of membrane envelopes and the thickness of element materials (feed spacer, permeate carrier, and membrane), the impact of trade-off between flow and rejection can be predicted and optimized.
Spiral wound elements are usually placed inside of a cylindrical pressure vessel for operation. While there are exceptions, makers of spiral wound elements and the vessel that enclose them have converged on a few standard dimensions. Nominal diameters of 50 mm, 60 mm, 100 mm and 200 mm are most common for RO/NF elements. Elements having 60 mm diameter are usually available as approximately 350 mm, 530 mm, or one meter long, measured along the axis from the ends of slightly extended permeate collection tubes. Elements with diameters of 100 mm or 200 mm are usually available only in a one meter length. Vessels are created to hold an integral number of these elements in series. One reason for these standard element lengths is that the industry has commonly made membrane with a width of approximately one meter and the stated lengths allow for efficient use of this membrane. The permeate carrier and feed spacer sheets can be also be cut efficiently from meter wide rolls. Assigning the axial dimension of a spiral wound element to be an integral fraction of one meter allows the length of individual membrane envelopes to be unconstrained by materials.
In commercial RO and NF applications, a large filtration system may be composed of more than 10,000 elements, usually distributed in pressure vessels containing 4 to 7 elements each. The pressure vessels have ports for inputting the pressurized feed solution and removing the concentrate and permeate solutions. Feed flows axially through each of the elements in series. By connecting the permeate collection tubes of different elements, the effect is to create one long element in a vessel. Each pressure vessel can be further combined in series or parallel with other vessels to create a filtration system. A filtration system can be operated with re-circulation where concentrate is re-pressurized and allowed to pass several times through a vessel or in ‘single-pass’ mode, where solution passes only once through any portion of the system. Large single-pass filtration systems are typically arranged in a tapered design where the concentrate from several upstream vessels feeds a smaller number of downstream vessels. While such systems may achieve high recovery with high cross flow, they are also characterized by a long continuous feed path and high pressure drop. System design can be further complicated by incorporating a variety of other options, including booster pumps, permeate pressurization, and cascading stages. Appropriate system design allows the desired recovery and permeate quality to be achieved, and many of the available options are described and illustrated in Marcel Mulder, “Basic Principles of Membrane Technology”, Chapter 8, Kluwer Academic Publishers, Dordrecht, The Netherlands, (1991).
Separation efficiency in RO or NF spiral wound elements is dictated by pressure and concentration gradients across the membrane. The flux (volumetric flow per unit area of membrane) of the solvent (most often water) is generally proportional to the net-driving pressure. This net-driving pressure is defined as the difference in feed and permeate applied pressures minus the osmotic pressure differential across the membrane. Solvent flux goes down for high solute concentration and low transmembrane pressure. By contrast, solute molecules commonly pass through RO and NF membrane based on diffusion and this process is ideally driven by a concentration gradient and not affected substantially by the pressure gradient. As a consequence, the concentration of a well-rejected solute in the permeate is inversely proportional to the net-driving pressure.
The net-driving pressure of an element can be influenced by the pressure drop between the fluid entrance end of a spiral wound element and the exiting end. In operation, a feed solution under pressure is applied to the entrance end of the spiral wound element and it flows axially through the cylindrical element. The resulting pressure drop depends on the volume of feed flowing through the element and the resistance of the feed spacer sheet to this flow. This pressure drop is less (usually much less) than the net-driving pressure. A typical pressure drop across a one meter long RO/NF element would be about 25 kPa when the feed has a superficial velocity of 15 cm/sec in the channel. The superficial velocity may be defined as the volumetric flow divided by the cross sectional area of an empty channel. For a standard commercially available Film Tec™ 200 mm diameter (8 inch) element with 35 m2 (380 ft2) of membrane, the 15 cm/sec corresponds to about 200 m3/day of feed solution. Pressure drop is approximately linear with flow rate.
In a series of elements such as in a vessel, the first element in series operates with higher net-driving-pressure than those at the downstream end resulting in an uneven distribution of flux. One cause for the difference is the fact that feed concentration increases in successive elements resulting in more osmotic pressure. This effect is augmented (and sometimes overshadowed) by the pressure drop down a series of elements. There are several problems with this inhomogeneous utilization of membrane. For the lead elements, high flux can substantially shorten the life of an element due to fouling and scaling. High flux also promotes concentration polarization and polarization decreases the effective rejection of the membrane. Lower flux in the trailing elements is also undesirable, not only because of decreased productivity, but also because lower flux means higher solute concentration in the permeate.
Pressure drop can result in a need for higher overall pressure requiring greater energy input and necessitating higher cost equipment (pumps, piping, vessels, etc.). To reduce pressure drop some vessels are manufactured with a port in the middle of the vessel allowing the feed solution to flow half as far in two directions. Alternatively, this problem can be addressed by the use of either booster pumps between vessels or by permeate pressurization of the lead elements. In any case, these remedial measures result in greater complexity and cost.
Ideally, the problem of pressure drop would be addressed at the source—the feed spacer sheet. Unfortunately, the selection of an optimal feed spacer sheet can be a complex task as several key aspects of the feed spacer sheet are difficult to predict from its structure. In addition to having low resistance to feed flow so as to maintain low pressure drop across the element, the ideal feed spacer would also have other characteristics. The primary purpose of the feed spacer is to separate two membrane sheets, allowing the feed solution to flow across their front surfaces. Towards that end, the ideal permeate spacer sheet (also referred to as “spacer”, “net” or “spacer sheet”) would have a high density of contacts points with the membrane so that nesting of membrane envelopes is avoided. Nesting results when the contact points of adjacent envelopes become offset during rolling and deformation of the two envelopes decreases a spacer's effective volume. Ideally, the spacer would be thin so that it does not greatly decrease the amount of membrane area that can be packed into an element of a given diameter; it would promote substantial mixing so that solute polarization at the surface of the membrane is small; it would have a smooth surface so as not to damage the discriminating layer of the membrane with which it is in contact; and of course it would also be inexpensive to manufacture and use.
Designing an optimal feed spacer is a balancing of competing concerns. For instance, increasing a spacer's thickness results in less pressure drop but conflicts with the desire to maximize an element's active membrane area. Another important conflict stems from the desire for reduced concentration polarization within an element. Concentration polarization is a phenomena resulting in a higher concentration of solute at the membrane's surface than in the bulk. It is caused by the membrane's selectivity. During operation, solutes in the feed solution are continually driven to the membrane surface by convective transport of the feed. In the absence of mixing, rejected solutes must be removed from the surface by diffusion. Coupling competing mass transfer processes to axial flow results in the solute concentration at the membrane's surface increasing down the length of the channel. The effect is especially important for large solute molecules, high permeate fluxes and low feed velocities in the axial direction. The increased concentration at the membrane's surface results in both decreased permeation of water (due to osmotic pressure, scaling, gel formation or fouling) and increased passage of solute molecules (caused by a greater effective concentration). One purpose of a feed spacer is to cause localized regions of turbulence, breaking up the build-up of polarization. Unfortunately, energy required for mixing at the membrane's surface must necessarily contribute to energy dissipation through the element (pressure drop).
In further regard to pressure drop, two key characteristics of the feed spacer are its thickness (height of the channel) and its void fraction. When the volume of liquid flowing through the spacer is kept constant, increasing either property will generally cause a decrease in pressure drop. The pressure drop of a “net-type” spacer oriented along the flow direction, as shown in FIG. 2, is further characterized in Da Costa, Fane, & Wiley, J. Membrane Science, 87, 79-98 (1994), where formulas recognize its dependence on several geometric characteristics: thickness, void fraction, mesh size, filament diameter, and the angle between filaments. The interrelations between these parameters are recognized and for a constant void fraction, the paper demonstrated the impact of trading mesh size and hydrodynamic angle. (The hydrodynamic angle is defined here as the angle, formed between two filaments, which faces the channel axis.) It was found that when flow, thickness, and void fraction are kept constant, decreasing the hydrodynamic angle resulted in a smaller pressure drop down the flow channel. At the same time a lower hydrodynamic angle (at the same void fraction) resulted in a greater mesh size and a dramatic decrease in mixing, parameterized by the mass transfer coefficient.
Mass transfer and pressure drop within spacers are still too complicated to compute accurately from first principals but several attempts have been made to estimate optimal feed spacer configurations, either analytically or by empirical study. The angle at which two filaments cross and their orientation relative to the feed flow are two related areas that have merited attention in the open literature, as both affect pressure drop and polarization.
In Da Costa, Fane, Fell, & Franken, J. Memb. Science, 62, 275-291 (1991), feed spacer sheets at different orientation were examined during the filtration of dextran through an ultrafiltration membrane. The study used several commercial feed spacers but extra configurations were obtained by varying their orientation in the channel or by removing cross strands to increase void fraction. It was found that the minimum operating cost was obtained with a feed spacer having a hydrodynamic angle of about 80°. A second study, Da Costa, Fane, & Wiley, (1994), more fully characterized these spacers according to geometric characteristics and developed a semi-empirical model for pressure drop that accounted for different sources of energy dissipation. The trade-off between pressure drop and mass transfer was detailed and the model was used to predict optimal net configurations. Predictions confirm the previous experimental results but also elaborated on the ranges of optimal angle and void fraction for ultrafiltration under different flow conditions. At low cross flow velocities it was concluded that a net-type spacer should combine low void fraction (about 0.4) with a hydrodynamic angle between 50° and 120°. A third study by the same authors (Da Costa & Fane, Ind. Eng. Chem. Res., 33, 1845-1851, (1994) found that the size and location of filaments positioned perpendicular to flow was particularly important to mass transfer. Under the conditions examined it was concluded that UF elements would have better mixing and would produce more flow when spacers composed of perpendicular filaments were oriented so that one set of filaments was perpendicular to the flow direction, as compared to when both sets of filaments were oriented at 45° to the channel axis.
In Polyakov & Karelin, J. Membrane Science, 75, 205-211, (1992), a different set of feed spacers were examined for filtration of sodium chloride through reverse osmosis composite membranes. The authors introduced a model for polarization that was dependent on the angle between filaments and the feed flow direction. It was hypothesized that regions between filaments corresponded to developing turbulent flow and that the periodic blockage of membrane by filaments caused regions of polarization attenuation. Based on examination of different spacers, including two that are similar to those used in commercial RO elements, they found the best configuration had a filament angle of 63.5° to the flow direction. This traversing angle is equivalent to a 127° hydrodynamic angle, as previously defined.
In Zimmerer & Kottke, Desalination, 104, 129-134, (1996) the authors examined flow through biplanar spacers formed by stacking two layers of grid rods at different angles. Using flow visualization techniques they characterized two extremes of flow types and related them to two parameters: the traversing angle and the dimensionless mesh size. (The dimensionless mesh size was defined as the mesh size divided by the filament diameter.) Channel flow was found to dominate when the angle was low and the mesh size was short, resulting in poor mixing. ‘Corkscrew flow’ dominated at the other extreme and it resulted in poor mass transfer between neighboring stream paths. The authors suggested that the two domains can be overlapped by appropriate choices for the angle and dimensionless wavelength allowing a “perfect mixing” that results in homogenous use of the membrane surface. A preferred spacer, based on this mixing criteria, had a hydrodynamic angle of 120° and a dimensionless mesh size of 5.5.
Toray Industries' JP 99235520 describes an element constructed from a web formed by crossing two set of overlapping filaments to result in a hydrodynamic angle between 30° and 80°. Working examples were based on a net-type spacer made with a hydrodynamic angle of 66°, net thickness of 0.7 mm, and a perpendicular spacing between strands of 2.7 mm. When water at 25° C. was passed through the net at 15 cm/s, a pressure gradient of about 46 kPa/m resulted. A related Toray application, JP 00042378, used the same examples and describes an element having a pressure drop of between 10 to 20 kPa when flow is 15 cm/s and between 30-40 kPa when flow is 25 cm/s.
Toray Industries' EP 1029583 is aimed at spacers having higher hydrodynamic angles. Elements were formed using a net of crossed filaments that has a hydrodynamic angle between 58° and 90°. The working examples at 66° appear to be the same as described in JP 99235520, resulting in a pressure gradient of about 46 kPa/m when operated at 25° C. and 15 cm/s. This disclosure also presents operational data for similar elements made with nets having hydrodynamic angles of 57.4° and 75.0°. There was no difference in measured pressure drops but the element falling outside of the ideal range with a hydrodynamic angle of 57.4° demonstrated lower salt rejection and lower flux.
Nitto Denko's JP 05168869 describes an element constructed using a net having one set of filaments parallel to the feed flow direction and the other set of filaments crossing the flow direction at an angle less than 80°. More preferably, this angle is between 20° and 50°. Examples provided use a 0.35 mm thick net having traversing angles of either 25° and 40°. The better of the two spacers demonstrated a pressure loss of about 76 kPa/m for a feed flow rate 15 cm/sec. The described spacer had the particular disadvantage of being asymmetric, so that the two surrounding membranes sheets each will see a different hydrodynamic environment. Also, as pointed out in EP 1029583, this spacer requires advanced net-making techniques when compared to the current art.
Feed spacers for commercial NF and RO elements have been characterized in several sources (e.g. G. Schock, A. Miquel, “Mass transfer and pressure loss in spiral wound modules, Desalination, 64, 339 (1987); S. V. Polyakov and F. N. Karelin, “Turbulence promoter geometry: its influence on salt rejection and pressure losses of a composite-membrane spiral wound module” J. Memb. Sci., 75, 205, (1992)). These are commonly made with a net-type feed spacer having an average thickness of between 0.5 and 2 mm, a perpendicular spacing between filaments of between 1 and 4 mm, a void fraction near 0.9, and a hydrodynamic angle about 90°. The net is oriented so that the flow direction bisects this angle, resulting in a traversing angle of 45°. In the Zimmerer paper, different configurations were studied through construction of spacer sections from stacked grid rods, using several hydrodynamic angles other than 90°. In the Da Costa articles, flow through nets having a variety of different configurations were examined by rotating or modifying existing netting. The Toray patent applications used webs having a variety of hydrodynamic angles. Several patents (U.S. Pat. No. 4,022,692, U.S. Pat. No. 4,861,487, U.S. Pat. No. 4,902,417) also describe low pressure-drop, asymmetric nets, having one filament set oriented parallel to the flow direction. Despite these studies, commercial manufacture of nets for RO/NF elements are still very dominated by the standard 90° netting and a 45° traversing angle, with limitations imposed by both convention and legitimate processing concerns.
Feed spacers for RO and NF elements are commonly formed by a process similar to that described in U.S. Pat. No. 3,067,084. This process begins with simultaneously extruding two sets of filaments to form a tubular biplanar net. In this process, two concentric dies are used, both having a multitude of orifices arranged in a circular pattern. One die rotates relative to the other during extrusion to cause filaments of one set to cross those of the other. The heated polymer filaments are caused to contact and join each other, either at the die face or shortly after existing the die, when they are still soft. The resulting tubular net, comprising the two sets of partially coalesced filaments, is then pulled and spread over an expansion mandrel. This further unifies the two sets of filaments, increases the diameter of the tube to satisfy end-use requirements, and decreases the weight and cost of the net on a unit-area basis.
Alternatively, tubular nets have been produced for elements by a process similar to that described in U.S. Pat. No. 3,700,521. In that case, a first die again extrudes polymer filaments through a plurality of openings arranged in a circle, as above. However, in this method, the second die opens periodically to extrude a full filament at one time, the filament being a continuous circle. The extrusion process brings the circular filament into contact with the others, where they are fused together. A variation of this process, with openings in the first die arranged in a line instead of a circle, is also amenable to directly extruding a flat net.
Either tubular netting structure may be slit along any axis to form a flat net, where crossing filaments form a two-dimensional array of parallelograms. When slitting is parallel to one set of filaments, a non-symmetrical net is formed that has crossing strands on only one side of the spacer. When the diameters and mesh sizes for the two sets of filaments are the same, the parallelograms are actually diamonds and a symmetric net can be achieved by slitting the tubular net parallel to a line through crossing points on opposite corners of a diamond. Intermediate cases exist as well, and the splitting cut determines the machine direction for the resulting flat net. When such a net is rolled up, the “machine direction” is perpendicular (orthogonal) to the axial direction of the roll.
The characteristic angle of a symmetric net is defined as the angle most open to the machine direction. This characteristic angle may be adjusted in the extrusion processes described above, although there are practical limits. Methods similar to those described in U.S. Pat. No. 3,067,084 allow for decreasing the characteristic angle over a reasonable range by increasing the rate at which the net is pulled away from the die and/or by decreasing the relative rotation rate between the two dies. However, at the other extreme, instabilities result at large characteristic angles because of the geometry of proximate filaments at the crossing point and the longer lengths of unsupported filaments approaching the mandrel. In the Conwed method, based on U.S. Pat. No. 3,700,521, the characteristic angle could, in principle, be varied by rotating the first die relative to the second. This would likely be prone to similar difficulties at large characteristic angle. However, the Conwed method is generally used for extruding nets having cross strands perpendicular (90°) to the extrusion direction.
Even more difficult to obtain is the combination of strand thinning and a large characteristic angle. Strand thinning, the stretching or necking of filaments between crossing points, can result from the tension placed on the net during manufacture. Some strand thinning is frequently seen in commercial nets used for RO/NF feed spacers. However, in the processes described above, machine tensions are typically in a direction that tends to decrease the characteristic angle. Difficulties inherent to inducing transverse orientation in the net by the above processes are described in U.S. Pat. No. 4,152,479, which is incorporated herein by reference.
Particularly for RO and NF applications, where small molecules are removed from a feed solution at low applied pressures, it is desired to have a feed spacer sheet with very low pressure drop. At the same time, it is desired that this feed spacer have a small strand spacing so nesting of adjacent layers is minimized and polarization remains acceptable. Further, such a feed spacer would need to be economical to produced and use. Most efficient use requires that the feed spacer's “machine direction” be perpendicular to the axis of the permeate collection tube and this imparts a constraint on the preferred orientation of nets during their production. It is one goal of this invention to provide an economical feed spacer characterized by very low pressure drop, small strand spacing and acceptable polarization, along with spiral wound elements incorporating the same.